1. Field of the Invention
The present invention relates generally to methods and apparatus for cardiac defibrillation, and more particularly to the use of biphasic waveforms of a specific configuration in conjunction with such methods and apparatus.
2. Relevant Background
Cardiac arrhythmias can arise in the atrial or ventricular chambers as a consequence of an impairment of the heart's electro-physiologic properties such as excitability, conductivity, and automaticity (rhythmicity). Tachycardia is an arrhythmia characterized by rapid beating of the affected chamber which, in some instances, may lead to fibrillation. In other instances, fibrillation may arise in a diseased heart without the advance episode of a tachyarrhythmia.
During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated random contractions, quickly resulting in a complete loss of synchronous contraction of the overall mass of tissue and a consequent loss of the blood-pumping capability of that chamber. Because of the lack of contribution of the atrial chambers to cardiac output, atrial fibrillation is hemodynamically tolerated and not generally regarded as life-threatening. However, in the case of ventricular fibrillation, cardiac output ceases instantaneously as a result of the rapid, chaotic electrical and mechanical activity of the excitable myocardial tissue and the consequent ineffectual quivering of the ventricles. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death will occur within minutes.
Since ventricular fibrillation is frequently triggered by acceleration of a ventricular tachycardia, various methods and devices have been developed or proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioversion. In the latter technique, the heart is shocked with one or more current or voltage pulses of generally considerably higher energy content than is delivered in pacing pulses. Unfortunately, the therapy itself presents a considerable risk of precipitating fibrillation.
Defibrillation--that is, the method employed to terminate fibrillation--generally involves applying one or more high energy "countershocks" to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to re-establish an organized spreading of action potential from cell to cell of the myocardium, thereby restoring the synchronized contraction of the mass of tissue. If these chaotic contractions continue in any tissue section, the defibrillation may be short-lived in that the uncontrolled tissue section remains a potential source for re-fibrillation. Successful defibrillation clearly requires the delivery of a shocking pulse containing a substantial amount of electrical energy to the heart of the afflicted person, at least adequate to terminate the fibrillation and to preclude an immediate re-emergence. Although high intensity defibrillation shocks are often successful in arresting fibrillation, they tend to precipitate cardiac arrhythmias, which themselves may accelerate into fibrillation. Moreover, the high intensity shocks can cause permanent myocardial injury.
In the conventional approach of transthrracic external defibrillation, paddles are positioned on the patient's thorax and, typically from about 100 to 400 joules of electrical energy is delivered to the chest area in the region of the heart. It is apparent that, from the manner in which the shock is applied, only a portion of this energy is actually delivered to the heart and, thus, is available to arrest fibrillation. Where fibrillation occurs during open heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium and, in these instances, the energy required to be delivered is considerably less, on the order of 20 to 40 joules.
More recently, implantable automatic defibrillators have been developed for use in detecting and treating ventricular fibrillation. In 1970, M. Mirowski et al. and J. O. Schuder et al. separately reported in the scientific literature their independent proposals for a "standby automatic defibrillator" and a "completely implanted defibrillator", respectively, including experimental results in dog tests. Since that time, a vast number of improvements in implantable defibrillators, including fibrillation detectors and high energy pulse generators with related electrode configurations, have been reported in the scientific literature and the patent publications.
The pulse energy requirements for internal defibrillation with known implantable defibrillators and electrode systems range from about 5 joules to approximately 40 joules. Of course, the actual energy level required may differ from patient to patient, and further depends on such factors as the type of pulse waveform and the electrode configuration employed. While advances and improvements in electrical energy sources in general and pacemaker batteries in particular have been made over the past few years, It is clear, nonetheless, that repeated delivery of such amounts of energy from an implanted system will deplete conventional batteries in relatively short order. Accordingly, for this and other reasons mentioned above, reduction of energy level required for internal defibrillation remains a key area of inquiry and investigation.
It is a principal object of the present invention to provide improvements in methods and apparatus for the generation and application of shocking waveforms effective for either internal or external defibrillation.
A related object is to provide methods and apparatus for generating and applying improved configurations of biphasic waveforms, usable preferably in conjunction with implantable automatic defibrillators but alternatively for external defibrillation. We have found these improved configurations to be effective in terminating fibrillation with delivery of considerably less energy than has been necessary using prior art systems and methods.
Prior proposed implantable defibrillators have commonly employed systems to produce unidirectional (also referred to as unipolar) shocking pulses, as for example, are described in U.S. Pat. Nos. Re. 30,372 to Mirowski et al., Re. 30,387 to Denniston et al., and 4,210,149 to Heilman et al. Some have suggested that the delivery of a sequence of unidirectional high intensity pulses by the implanted defibrillator is somewhat more effective. More recent studies have indicated that bidirectional (or biphasic) waveforms may decrease required defibrillation shock strengths and reduce post-shock cardiac arrhythmias. In the latter respect, exemplary publications include those of J. L. Jones et al., "Improved defibrillator waveform safety factor with biphasic waveforms," Am J Physiol 245 (Heart Circ Physiol 14): H60, 1983; "Decreased defibrillator-induced dysfunction with biphasic rectangular waveforms," Am J Physiol 247 (Heart Circ Physiol 16): H792, 1984; and "Reduced excitation threshold in potassium depolarized myocardial cells with symmetrical biphasic waveforms," J Mol Cell Cardiol 17: XXVII, 1985; and those of J. C. Schuder et al., "Transthoracic ventricular defibrillation in the 100 kg calf with symmetrical one-cycle bidirectional rectangular wave stimuli," IEEE Trans Biomed Eng 30: 415, 1983; and "Defibrillation of 100-kg calves with asymmetrical, bidirectional, rectangular pulses," Cardiovasc Res 419, 1984.